1) Field of the Invention
The present invention relates to a technology for obtaining an optical delay circuit having a compact size and stabilization against an environmental variation with an easy control, and an integrated optical device using the optical delay circuit.
2) Description of the Related Art
Conventionally, an optical signal time-division-multiplexed transmission system using an optical delay line has been proposed to increase a transmission capacity of the optical signal time-division-multiplexed transmission (see, for example, Japanese Patent Application Laid-Open Publication No. H6-53936). FIG. 19 is a schematic of a conventional optical signal time-division-multiplexed transmission system using the optical delay line. On a transmission side, n-channels of electrical signals are simultaneously input to an electrical-optical conversion circuit 111 in parallel to generate an n-channel optical signal. The n-channel optical signals are simultaneously input to n lines of optical delay line group 112 and an optical multiplexer 113 in parallel. The optical delay line group 112 has delay amounts sequentially different by a reference delay time corresponding to a time width of a time slot of each of the channels in a time division multiplexing. The optical multiplexer 113 multiplexes an output optical signal from each of the optical delay lines. Then, a time-series optical signal constituted with the n-channel optical signals delayed by the reference delay time is output to an optical transmission line 104 as a multiplexed output.
On a reception side, an optical demultiplexer 115 demultiplexes the n-channel time-series optical signal input from the optical transmission line 104 to n channels of optical signals. The n channels of the optical signals demultiplexed are simultaneously input to n lines of optical delay line group 116 in parallel. Then, output optical signals from the optical delay line group 116 are converted into n-channels of electrical signals by an optical-electrical conversion circuit 117.
The optical delay lines (112, 116) used in the optical signal time-division-multiplexed transmission system described in Japanese Patent Application Laid-Open Publication No. H6-53936 is configured using an optical fiber. With this conventional optical delay lines (112, 116), to generate a 1-bit delay Δt (=25 pico seconds (ps)) in transmission and reception of an optical signal of 40 gigabits per second (Gbps) between two split optical fibers, for example, it is necessary to provide a difference between optical fiber lengths by Δt×c/n=25 [ps]×3×1018 [meter (m)/second (sec)]/1.5≈5 [millimeter (mm)].
This type of optical delay line is also used to form a Mach-Zender interferometer. FIG. 20 is a schematic diagram for illustrating a conventional differential-phase-shift-keying (DPSK) reception device. As shown in FIG. 20, the DPSK reception device 150 includes a Mach-Zender interferometer 130 and a photodetector module 140 that are connected via an external optical fiber 151. The Mach-Zender interferometer 130 includes 1-bit delay line including two of multiplexer/demultiplexer 131 and an optical fiber 132. The photodetector module 140 includes two of photodetector 141. Thus, a time-delay optical circuit is formed (see, for example, Japanese Patent Application Laid-Open Publication No. H6-53936).
However, a device in which the optical delay circuit, which includes the conventional optical delay line, is provided becomes bulky, resulting in difficulty in handling the device because the optical delay line is required to be configured in such a manner that optical fibers have differences in length therebetween. Furthermore, to stabilize a temperature of the optical delay circuit, a large-capacity temperature controlling mechanism is necessary, and this makes the optical delay circuit to be installed in a small device.
In addition, with the DPSK reception device described in E. A. Swanson, et al., “High Sensitivity Optically Preamplified Direct Detection DPSK Receiver with Active Delay-Line Stabilization”, IEEE Photonics Technology Letters, 1994, 6, p. 263, because the optical fiber to connect with the Mach-Zender interferometer is externally attached to the photodetector, it causes a handling problem. Moreover, to stabilize a temperature of the DPSK reception device, a temperature control of the whole DPSK reception device is required, which results in a difficulty in downsizing the device. Besides, a combination of the device and the optical fiber leads to increased cost.